Thermal flow measuring devices are widely applied in process measurements technology. Corresponding field devices are manufactured and sold by the applicant, for example, under the marks, t-switch, t-trend or t-mass. The underpinning measuring principles are known from a large number of publications.
Typically, a flow measuring device of the field of the invention includes at least two sensor elements, of which each has a temperature sensor embodied as equally as possible. At least one of the sensor elements is embodied heatably. In this regard, a sensor element can contain a supplemental resistance heater. Alternatively, however, the temperature sensor can also be embodied as a resistance element, e.g. in the form of an RTD resistance element (Resistance Temperature Detector), especially in the form of a platinum element, such as also commercially obtainable under the designations PT10, PT100, and PT1000. The resistance element, also referred to as resistance thermometer, is then heated via conversion of electrical power supplied to it, e.g. as a result of an increased electrical current supply.
Often, the temperature sensor is arranged within a cylindrical shell, especially a shell of metal, especially stainless steel or Hastelloy. The shell functions as a housing, which protects the temperature sensor, for example from aggressive media. In the case of the particular at least one heatable temperature sensor, it must additionally be assured that a best possible thermal contact is provided between the heatable temperature sensor and the shell.
For registering the mass flow and/or the flow velocity, the at least two sensor elements are introduced into a pipeline, through which flows, at least at times and at least partially, a flowable medium. The sensor elements are in thermal contact with the medium. They can, for this, either be integrated directly into the pipeline, or into a measuring tube installable in an existing pipeline. Both options are subject matter of the present invention, even when in the following only a pipeline is discussed.
In operation, at least one of the at least two temperature sensors is heated (the active temperature sensor) while the second temperature sensor remains unheated (the passive temperature sensor). The passive temperature sensor is applied for registering the temperature of the flowable medium. The terminology, temperature of the medium, means, in such case, the temperature, which the medium has without an additional heat input of a heating unit. The active sensor element is usually either so heated that a fixed temperature difference is established between the two temperature sensors, wherein the change of the heating power is taken into consideration as measure for the mass flow and/or the flow velocity. Alternatively, however, also the fed heating power can be kept constant, so that the corresponding temperature change is taken into consideration for determining the mass flow and/or the flow velocity.
If no flow is present in the pipeline, the removal of the heat from the active temperature sensor within the medium occurs via heat conduction, heat radiation and, in given cases, also via free convection. For maintaining a certain temperature difference, then, for example, a constant amount of heat is required as a function of time. In the presence of a flow, in contrast, there is an additional cooling of the active temperature sensor from the flow of the flowing, colder medium. An additional heat transport occurs due to forced convection. Correspondingly, thus, as a result of a flow, either an increased heating power must be supplied, in order to maintain a fixed temperature difference, or else the temperature difference between the active and passive temperature sensors lessens.
This functional relationship between the heating power supplied to the active temperature sensor, or the temperature difference, and the mass flow and/or the flow velocity of the medium through the pipeline can be expressed by means of the so-called heat transfer coefficient. The dependence of the heat transfer coefficient on the mass flow of the medium through the pipeline is then used for determining the mass flow and/or the flow velocity. Along with that, the thermophysical properties of the medium as well as the pressure reigning in the pipeline have an influence on the measured flow. In order also to take into consideration the dependence of the flow on these variables, the thermophysical properties are, for example, furnished within an electronics unit of the flow measuring device in the form of characteristic curves or as parts of functional, determinative equations.
It is not possible by means of a thermal, flow measuring device to distinguish directly between a forwards directed and a backwards directed flow. In such case, the terminology, flow direction, means, here the macroscopic flow direction, such that partially occurring vortex or directional deviations are not taken into consideration. If the flow direction is not known, then especially in the case of flow not constant as a function of time or also very small flows, considerable measurement errors can disadvantageously be experienced in the determining of mass flow and/or flow velocity.
Various thermal, flow measuring devices have been developed and disclosed, which have, besides the determining of mass flow and/or flow velocity, a supplemental functionality for flow direction detection. For ascertaining the flow direction, frequently utilized is the fact that different local flows, which directly surround the particular sensor element, lead to different cooling rates of the respective sensor element in the case of respectively equal supplied heating power. Different local flows can be implemented, for example, by integrating a bluff body into the pipeline in the direct vicinity of at least one of the at least two sensor elements, by a non-equivalent arrangement of the at least two sensor elements with reference to the flow profile, or also by different geometric embodiments of the at least two sensor elements.
In the case of the flow measuring device of German patent publication, DE102010040285A1, for example, a plate is arranged within a measuring tube on a connecting line between a first and a second heatable temperature sensor. Based on comparison of so-called decision coefficients, which result from the respective heating powers and temperatures of the at least two heatable temperature sensors, then the flow direction of the medium is ascertained. These decisions coefficients are likewise taken into consideration in German patent publications DE102009045956A1 and DE102009045958A1 for determining the flow direction. In such case, the flow measuring device of German patent publication DE102009045956A1 includes a flow guiding body, which is arranged together with a heatable temperature sensor in a line essentially parallel to the pipeline axis, and a further temperature sensor is arranged spaced therefrom. In contrast, in the case of the flow measuring device of German patent publication DE102009045958A1, at least two heatable temperature sensors are arranged in two sleeve sections, and the at least two sleeve sections point in at least two directions with reference to the measuring tube axis.
In German patent publication DE102007023840B4, a thermal, flow measuring device with flow direction detection is described, which includes at least three sensor elements, whereof two sensor elements are arranged one after the other in the flow direction and at least one of these two sensor elements is heated, and at least at times in reference to the flow direction the heated sensor is arranged in front of the non-heated sensor element, and at times the non-heated sensor is arranged in front of the heated sensor element. The third sensor element is, furthermore, periodically temporarily heatable and arranged outside the flow across the first two sensor elements. The deviation of the respectively won measured values is then a measure for the flow direction of the medium.
Other causes for an, in given cases, considerable, measured value corruption lie, for example, in a change of the thermal resistance of at least one of the utilized sensor elements, which can lead to a change of the heat transfer from the heating unit into the medium in the case of otherwise constant flow conditions. Such a change of the thermal resistance is also referred to as sensor drift. In given cases, when the change of the effective thermal resistance remains below a certain predeterminable limit value, and in case the change is detected, the sensor drift as well as the negative influence on the determining of the mass flow and/or the flow velocity can at least partially be removed by suitable countermeasures. Otherwise, in given cases, the flow measuring device must at least partially be replaced.
Fundamentally with reference to the thermal resistance, a distinction is made between an inner thermal resistance and an outer thermal resistance. The inner thermal resistance depends, among other things, on individual components within the sensor element, e.g. within the sleeves. Thus, sensor drift can arise from defects in solder connections due to tensile loads from material expansion or the like. The outer thermal resistance is, in contrast, influenced by accretion, material removal or material transformation (e.g. corrosion) on the surfaces of the respective sensor element contacting the medium. A change of the outer thermal resistance is, thus, especially relevant in the case of long periods of operation and/or contact with aggressive media. In the case of gaseous- or vaporous media, the measuring of mass flow or flow velocity can, moreover, also be degraded by condensate formation on at least one of the temperature sensors.
Known from the state of the art are a number of flow measuring devices, by means of which a diagnosis concerning at least one of the sensor elements can be actuated. Thus, information concerning the state of at least one of the sensor elements is provided.
German patent publication DE102005057687A1 describes a thermal, flow measuring device having at least two heatable temperature sensors, wherein the first temperature sensor and the second temperature sensor are alternately operable as a passive, non-heated temperature sensor, which during a first measurement interval provides information concerning the current temperature of the medium, and as an actively heated temperature sensor, which during a second measurement interval provides information concerning the mass flow of the medium through the pipeline. A control/evaluation unit issues a report and/or undertakes a correction, as soon as the corresponding measured values of the two temperature sensors provided during the first measurement interval and the second measurement interval deviate from one another. In this way, accretion and condensate formation can be recognized.
Similarly, disclosed in German patent publication DE102007023823A1 is a thermal, flow measuring device having two, phasewise alternately heatable sensor elements as well as method for its operation. The mass flow is, in such case, alternately ascertained based on the respectively heated sensor element, wherein the respectively non-heated sensor element is referenced for ascertaining the temperature of the medium. From a comparison of the measured values with the two sensor elements, supplementally, a fouling of at least one of the two sensor elements can be detected.
Described in U.S. Pat. No. 8,590,360 B2 is the heating or cooling of a first heatable sensor element with a first heating power, and simultaneously the heating or cooling of a second heatable sensor element with a second heating power. Typically, the two heating powers are so selected that the temperatures of the two sensor elements differ. Through a comparison of the temperature of the medium, and/or of at least two independent variables characterizing the heat transfer coefficient, then a diagnosis can be made concerning the flow measuring device.
Finally, known from published international patent application, WO/2008/142075A1 is a method, in the case of which the heatable temperature sensor in thermal contact with a medium is heated with an alternating electrical current- or voltage signal and the produced heat at least partially given off to the flowing medium. The course of a heat- and/or cooling process occurring within the temperature sensor is measured and therefrom the state of the temperature sensor, especially a fouling and/or an accretion thereof, is diagnosed. At the same time, the mass flow can be determined.
In principle, the described flow measuring devices with a diagnostic function detect a change of the thermal resistance. Based on that, then the presence of accretion—and/or condensate formation can be deduced. This corresponds to a change of the external thermal resistance. As described above, a sensor drift can, however, also be brought about by a change of the inner thermal resistance. The inner thermal resistance is determined by the construction and materials used within the respective sensor element, especially by the different components e.g. within the sleeves, or housing, or by various material connections and/or—transitions, such as e.g. soldered connections. It would thus be desirable to provide a diagnostic function, by means of which, in the case of a sensor drift in at least one of the at least three sensor elements, it could be distinguished between a change of the outer thermal resistance and the inner thermal resistance.
Most of these flow measuring devices, which are suited for diagnosis of an accretion- and/or condensate formation or for providing information concerning the state of at least one sensor element, are, however, not able, simultaneously, to ascertain the flow and the diagnosis, or, simultaneously, the flow and the flow direction, or both. In the case of the respectively applied measuring principles, the individual sensor elements are at times heated, and serve at times for registering the temperature of the medium. Correspondingly, at each change of the temperature for one of the sensor elements, it is necessary to wait for the next measured value registering until the new temperature has, in each case, become stable. Thus, for example, the mass flow and/or the flow velocity cannot be continuously determined. Correspondingly, the methods assume an almost constant flow rate of the medium through the pipeline, at least during the time required for reaching a stable new temperature by at least one sensor element. However, it is in practice frequently the case that the flow rates vary at least slightly with time, which then can lead to a corrupted measurement result. This is especially problematic in the case of high flow rates.